For the sequencer, what I'd like to do is to build something that functions similarly to the microfreak's internal sequencer but with more parameters, the possibility for much longer recordings, and some cool options for track changes. Once I'm happy with the patch and the score, I'd like the program to then play the tracks and record them for later mixing. After that I'd change the patch and build additional instrument scores with the same method.
I'd also like to be able to visualize the contents of the tracks in a particular way.
I think Cinder will probably be sufficient for building the UI and doing the audio recording and mixing. I'll probably write my own MIDI layer, and I'll probably bolt on Lua for implementing the sequencer's high level logic.